1. Field of the Invention
The present invention relates to a solid-state laser apparatus, and more specifically to a small soliton mode-locked solid-state laser apparatus capable of short-pulse operation.
2. Description of the Related Art
Solid-state lasers in which a semiconductor laser (LD) is used as the excitation light source and a solid-state laser medium (laser crystal, ceramics, glass, or the like) doped with rare earth ions or transition metal ions is excited by the light source have been actively developed. Among them, short pulse lasers that generate so-called short pulse beams in the range from picoseconds to femtoseconds have been sought and proposed in many application fields including medicine, biology, machine industry, and measurement, and some of them are put into practical use after demonstration.
This type of laser apparatus generates short pulses by so-called mode locking. To put it briefly, the mode locking is a phenomenon in laser oscillation where all phases of multi-longitudinal modes are locked (relative phase difference=0) in the frequency domain, and the pulses become extremely short in the time domain due to multimode interference between longitudinal modes. In the field of solid-state laser apparatuses, mode locking by semiconductor saturable absorbing mirror (SESAM) has been actively developed since it is simple, low cost, small, and self initiating.
Particularly, in soliton mode locking, which is one of the CW mode locking regimes, the combination of negative group velocity dispersion in the laser resonator and self phase modulation mainly in the laser medium enables generation of pulses in the femtosecond region. More specifically, the soliton mode locking is a regime in which mode locking is initiated and the pulses are maintained/stabilized by the semiconductor saturable absorbing mirror, at the same time the mode locked pulses become sharp through soliton pulse forming which occurs by balancing the negative group velocity dispersion with self phase modulation, thereby stable pulse generation becomes possible (Optics Letters, vol. 25, No. 15, pp. 1119-1121, 2000 (Non-Patent Document 1), and Journal of Optical Society of America, vol. 16, No. 1, pp. 46-56, 1999 (Non-Patent Document 3).
Basically, a solid-state laser apparatus that realizes the soliton mode locking includes a solid-state laser medium, a saturable absorbing mirror, and a negative group velocity dispersion element within the resonator. In the following, the negative group velocity dispersion is also simply referred to as negative dispersion.
A typical configuration of a conventional soliton mode-locked solid-state laser doped with Yb (solid-state laser medium is Yb:KGd(WO4)2) as described in Non-Patent Document 1 is shown in FIG. 17. In the drawing, the reference symbol 80 is an excitation light source that emits, for example, 980 nm excitation light, 81 is an input optical system provided for each of a pair of the excitation light sources 80, 83 is a solid-state laser medium, M1 and M2 are a pair of concave mirrors forming a resonator with a curvature radius of, for example, 20 cm, 84 is a concave mirror with a curvature radius of 20 cm, 85 is a SESAM, 86 and 87 are prisms forming a prism pair made of, for example, SF10 glass, 88 is a knife edge plate, and 89 is an output coupler with a transmittance of, for example, 4.3%.
As illustrated in FIG. 17, the conventional apparatus employs a configuration in which the beam radius of the laser oscillation light is focused separately at the solid-state medium and SESAM by the concave mirrors M1, M2, and 84, in order to reduce the beam radius ωL at the laser medium and the beam radius ωA at the SESAM.
The spot sizes (beam radius of the oscillation light) on the laser medium and SESAM are reduced for the following two reasons. First reason is to reduce the threshold value of laser oscillation, and the second reason is to satisfy the soliton mode locking condition.
The first reason will now be described. The laser oscillation threshold Pth is represented by Formula (1) below as described, for example, in Applied Optics, Vol. 36, No. 9, pp. 1867-1874, 1997 (Non-Patent Document 2).
                              P          th                =                                            π              ⁢                                                          ⁢              h              ⁢                                                          ⁢                                                ν                  P                                ⁡                                  (                                                            ω                      L                      2                                        +                                          ω                      P                      2                                                        )                                                                    4              ⁢                                                          ⁢                                                στη                  a                                ⁡                                  (                                                            f                      1                                        +                                          f                      2                                                        )                                                              ⁢                      (                                          L                i                            +                              T                OC                            +                              2                ⁢                                  f                  1                                ⁢                σ                ⁢                                                                  ⁢                                  N                  0                                ⁢                ls                                      )                                              (        1        )            
where, ωP is the excitation light beam radius at the solid-state medium, hνp is the excitation light photon energy, σ is the cross-section of stimulated emission of the solid-sate laser medium, τ is the upper level life time, ηa is the absorption efficiency, f1 is the lower level occupancy, f2 is the upper level occupancy, Li is the intracavity loss of the resonator, Toc is the output mirror transmittance, N0 is the rare earth ion dope density, and ls is the crystal length.
Formula (1) above shows that it is only necessary to reduce the oscillation beam radius ωL and excitation light beam radius ωP in the solid-state laser medium in order to reduce the oscillation threshold.
Next, the second reason, that is, the soliton mode locking condition will be described. As described in Non-Patent Document 3, it is known that mode locking mixed with Q-switching operation (Q-switched mode locking) occurs in a soliton mode-locked laser apparatus under a certain condition. The Q-switched mode locking is an operation mode in which a mode-locked pulse train (frequency in the range from 10 MHz to 1 GHz, pulse width in the range from picoseconds to femtoseconds) is disposed in a long pulse of Q-switched pulse (frequency in the range from 1 KHz to several 100 KHz, pulse width in the range from microseconds to nanoseconds).
This operation mode is generally undesirable other than for energy application due to instability in output, pulse width, and pulse period. According to the Non-Patent Document 3, the condition not to cause Q switching in the soliton mode locking using a saturable absorption mirror are expressed by Formula (2) below.Fsat,L·Aeff,L·g·K2EP3+EP2>Fsat,L·Aeff,L·Fsat,A·Aeff,A·ΔR  (2)
where: Ep is the intracavity pulse energy; ΔR is the modulation depth of the saturable absorption mirror; Fsat,A is the saturated fluence of the saturable absorption mirror; Fsat,L (=hν/σ) is the saturated fluence of the laser medium; hν is the oscillation laser photon energy; Aeff,A (=πωA2) is the oscillation light beam cross section at the saturable absorption mirror; Aeff,L(=πωL2) is the oscillation light beam cross section at the laser medium, g is the laser gain of the laser medium, and K is the following.
  K  =                    4        ⁢                                  ⁢        π        ⁢                                  ⁢                  n          2                ⁢        ls                                        D                          ⁢                  A                      eff            ,            L                          ⁢                  λ          0                ⁢                  Δν          G                      ⁢          0.315      1.76      
(where, n2 is the nonlinear refraction index of the laser medium, D is the group velocity dispersion of the entire resonator for one round-trip (D<0), λ0 is the center frequency of the oscillation light, and ΔνG is the gain bandwidth.) Note that, in Formula (2) above, the solution of Ep when the left term corresponds to the right term is the mode locking threshold, and to satisfy Formula (2) means to set the Ep to a value greater than the mode locking threshold.
Formula (2) shows that it is necessary to reduce the beam cross section at the laser medium and the beam cross section at the SESAM, and/or to increase the intracavity pulse energy Ep in order to avoid generation of the Q-switched mode locking.
Because of the aforementioned reasons, most of conventional apparatuses like that shown in FIG. 17 employ a configuration in which a laser medium is placed between two concave mirrors to narrow down the beam, and at the same time beam focusing is performed on the SESAM by a concave mirror.
In the mean time, U.S. Pat. No. 7,106,764 (Patent Document 1) and Japanese Unexamined Patent Publication No. 11 (1999)-168252 (Patent Document 2) propose configurations to downsize mode-locked solid-sate laser apparatuses having a negative group velocity dispersion element in the resonator. More specifically, Patent Document 1 proposes a mode-locked solid-state laser apparatus downsized by disposing the solid-state laser medium and SESAM in close proximity to each other, while Patent Document 1 proposes a mode-locked solid-state laser apparatus in which a saturable absorption mirror is formed on a solid-state laser medium by coating and a negative dispersion mirror doubles as the output mirror to reduce the number of optical components and to downsize the apparatus.
The conventional mode-locked solid-state laser apparatus having the configuration described above and illustrated in FIG. 17 requires at least three concave mirrors, and in some cases further require a plurality of mirrors for beam replication. This increases the number of optical components of the mode-locked solid-state laser, and the apparatus cost is increased.
Here, the size of a conventional mode-locked solid-state laser apparatus is discussed more specifically. The distance between the concave mirror and the laser medium, and between the concave mirror and SESAM is usually set to a value about half the curvature radius, which alone amounts to about 150 mm (when curvature radius is 100 mm) to 300 mm (when curvature radius is 200 mm) in total. Further, when considering an insertion space for a negative dispersion element or the like, the resonator requires a length of about 500 mm to 1 m, so that the laser apparatus inevitably become large. Generally, when a resonator of a meter length is formed in a solid-state laser, stable operation is difficult. For this reason, the conventional apparatuses have low laser oscillation stability.
Further, the laser apparatus includes a complicated excitation optical system formed of a combination of a plurality of lenses for sufficiently focusing laser light emitted from the excitation light source, such as LD or the like, before inputting to the solid-state laser medium in order to reduce the beam radius ωp in the solid-state laser medium, thus the excitation optical system tends to be large.
Patent document 1 and Patent Document 2 describe example resonators downsized through linear configuration, but the excitation optical system is structured so as to input excitation light through a concave mirror, as in the example described above, therefore downsizing of the excitation optical system is not sufficient.
In view of the circumstances described above, it is an object of the present invention to provide a small, low cost, and highly stable solid-state laser apparatus capable of realizing femtosecond CW mode locking.